Step 1

Heat the oven to 180ºC fan/gas Lightly butter a large 2.5 litre baking dish or casserole. Put the potatoes and onion in a large saucepan, cover with cold water and bring to the boil. Simmer for 15 minutes until the potatoes are very tender.

Step 2

Drain the cooked potatoes and onion, cover with a clean tea towel and set aside to steam dry for 4-5 minutes. Mash the potato mixture, then season to taste with salt and pepper. Add the 75g butter, mustard and enough warm milk to loosen the mixture to a light, fluffy consistency, then beat in the eggs.

Step 3

Add three-quarters of the grated cheese to the mash and mix well. Spoon the cheese and potato mixture into the prepared dish, then sprinkle the remaining cheese over the top.

Step 4

Bake for 20 to 30 minutes or until the pie is steaming hot and the top is golden brown. Serve immediately, garnished with chives if you like, with steamed cabbage or salad leaves.